Sharpshooter electropenetrography X wave represents the Xf inoculation behaviors: Update on evidence from systemic, symptomatic Pierce’s disease infection induced after X waves

Authors: Backus, Elaine; Cervantes, Felix; Van De Veire, Jacqueline - Jackie; Burbank, Lindsey; PERRING, THOMAS - University Of California

Submitted to: CDFA Pierce's Disease Control Program Research Symposium
Publication Type: Proceedings
Publication Acceptance Date: 10/23/2018
Publication Date: 11/24/2018
Citation: Backus, E.A., Cervantes, F.A., Van De Veire, J.A., Burbank, L.P., Perring, T. 2018. Sharpshooter electropenetrography X wave represents the Xf inoculation behaviors: Update on evidence from systemic, symptomatic Pierce’s disease infection induced after X waves. CDFA Pierce's Disease Control Program Research Symposium. available: https://www.cdfa.ca.gov/pdcp/Documents/Proceedings/2018ResearchProgressRpts.pdf..

Interpretive Summary:

Technical Abstract: The mechanism of Xf inoculation by sharpshooter vectors has been hypothesized for nearly 50 years to be due to egestion from the functional foregut. Recently, combined egestion plus salivation was conclusively demonstrated as the mechanism of bacterial ejection from the stylets. However, bacteria were ejected into/onto artificial diets, not plants. It has been hypothesized that: 1) egestion plus salivation eject Xf bacterial cells into grape xylem cells, and 2) electropenetrography (EPG) can be used to observe and quantify these vector behaviors in real time because the sharpshooter X wave represents combined salivation and egestion. Once these hypotheses are definitively supported, EPG can be used to search for novel avenues of disease resistance. This report updates progress in analyzing data from a 4-year project to conclusively test these hypotheses. In 2015, clean blue-green sharpshooters were allowed to feed on artificial diets containing ca. 107 Xf cfu for 2.5 to 7 hours. Each insect was then allowed to make a single, EPG-recorded stylet probe into a small grapevine petiole. Two treatments were used: 1) feeding was interrupted before a mature xylem vessel was reached and XN performed, and 2) the insect was allowed to reach a mature xylem cell and perform two to four XN events. qPCR was used to detect bacteria in the plants from the two treatments. Bacteria were detected in 19 (28%) of the 67 insect-probed plants, with 95% of those positive plants from the X wave treatment. Thus, results to date support the hypothesis that the XN portion of the X wave represents the Xf inoculation behaviors, and that (under some circumstances), a single stylet probe with at least two XN events can initiate a qPCR-positive, systemic (both close to and distant from the site of probing), symptomatic Pierce’s disease infection. If future tests continue to support the hypothesis, then EPG can be used to identify grapevine resistance to vector inoculation behaviors.